This invention relates to suppressing offensive oscillations, such as pressure oscillations in gas turbine combustors, by means of a minimum-seeking phase selection for a compensating modulation of a process adjusting input variable, such as fuel flow.
In axial flow gas turbine engines, combustion instability occurs when acoustic waves in the combustion chamber couple with some other physical phenomena, such as heat release or vortex shedding, and results in high pressure oscillations. Such oscillations cause vibration of combustor components which results in fatigue which can lead to reduced cycle life or unexpected catastrophic failure. This form of combustion instability also causes high pressure levels in thrust augmenters, such as military engine afterburners. The problems with combustion instability become significant in lean premix gas turbine engines which may be required in order to meet increasingly low emission level regulations promulgated by governments.
The combustion process involves chemical reactions, unsteady fluid motion, and heat transfer, all coupled in a non-linear way. Therefore, the combustion process is so extremely complex that any reasonably accurate model would involve a coupled system of non-linear partial differential equations which would prohibit direct analysis of the dynamics and on-line control thereof.
An attempted solution presented in U.S. Pat. No. 5,784,300 involves an exhaustive, unidirectional search of the entire parameter space, looking for optimal tuning. Because the increments of gain must be kept sufficiently small so as to not miss a region with good parameter values, the search is extremely slow. Since the phase may go through a change of close to 360xc2x0, if the initial value is only slightly off of the optimal value, the controller may well drive the system through regions where positive feedback further amplifies the offensive oscillations, causing closed-loop performance to be worse than open loops uncontrolled system operation.
Other processes have similar operating problems.
Objects of the invention include: fast automatic tuning of control parameters of processes such as combustion chamber dynamics; control of the dynamics of combustion chambers and other processes in a manner which will not excite the oscillations (not positive feedback); control of combustor pressure dynamics in a way to support utilization of lean premixed gas turbine engines;
This invention is predicated in part on our discovery that the pressure magnitude dynamics in a combustion chamber is separated in time scale from other dynamic processes, so that the pressure magnitude dynamics may be treated as the slowest process. This invention is further predicated on our discovery that, for a controller with fixed gain, the pressure magnitude as a function of a trimming fuel valve control phase has a periodic, roughly sinusoidal shape, with a unique minimum. The invention is predicated also on our discovery that use of a frequency tracking observer provides on-line control of phase shift feasible for counteracting a changing pressure dynamic in a combustor.
According to the present invention, a frequency tracking observer, such as a frequency tracking extended Kalman filter, responsive to a process parameter, such as combustor pressure, produces in-phase and quadrature components of the estimated magnitude of the undesirable variations in the parameter, such as combustor pressure variations, for which compensation is to be achieved; a bidirectional minimum-seeking algorithm is used to select the phase of a process adjusting input variable, such as fuel that is in addition to the main fuel flow used for power control purposes. The invention may be used to control any actuation mechanism that affects the level of pressure oscillations and allows parameter update in a scale faster than that of the operating conditions and slower than that of the dynamics being regulated, to suppress pressure oscillations or other parameters.
The invention reduces pressure oscillations in an axial flow gas turbine engine by on the order of fifty percent or more. The invention may be utilized to achieve acceptable pressure oscillations while achieving low emissions attendant lean premix gas turbine engines.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.